The present invention relates to the use of rubber compositions intended for the manufacture of tires as elastomeric internal filling compositions, also referred to as xe2x80x9ccushion mixesxe2x80x9d, in the zones of the shoulder and the crown of a tire.
Radial-carcass tires for motor vehicles bearing heavy loads at greater or lesser speeds, in particular those for heavy vehicles, have a framework formed of reinforcements or plies of metal wires coated with elastomers. Such tires comprise, in the bottom zone, one or more bead wires and carcass reinforcement extending from one bead wire to the other and, at the crown, a crown reinforcement comprising two or more crown plies. This framework is consolidated by elastomeric compositions. Radial-carcass tires, intended to be fitted on vehicles bearing heavy loads at greater or lesser speeds, in particular those for heavy vehicles, are designed to be able to be recapped several times when the tread that is in contact with the ground is worn. This involves having available recappable carcasses which have not been subject to serious damage after wear of one or more treads.
The life of the tire can be shortened due to the appearance of damage within a rubber profiled filling member, for example a break, which may then spread as far as the inner or outer surface of the tire, with the result that the tire cover must be discarded and replaced. Examples of damage are, at the level of the shoulder of the tire, a break in the rubber profiled member of triangular shape, separating the carcass ply reinforcement from the radially inner crown ply, said break resulting from an imposed deformation stress, such as an impact against a curb or an impact against the edge of a roundabout located at a cross roads, because some roundabouts are too cramped for a highway unit With trailer to be able to pass without mounting an edge, the profile of which is frequently harsh.
It is desirable for the cohesion of the rubber internal filler mixes to be as great as possible to avoid or reduce these incipient points for damage.
It is known to the person skilled in the art that elastomeric internal filler compositions undergo deformation upon each rotation of the wheel. Such deformation causes a great amount of heating which is harmful to the life of said compositions because, at operating temperatures which are frequently above 100xc2x0 C., the mechanical properties and the reinforcement degrade over time by thermochemical and/or thermo-oxidizing aging, with the consequence that the compositions become less resistant to mechanical stress.
In order to eliminate or at the very least minimize as far as possible the risks of breaking of the elastomeric internal filler mixes, i.e., those devoid of reinforcements, it is desirable for these mixes to have high mechanical cohesion as well as hysteresis loss characteristics which are as low as possible at the operating temperature of the tire.
The person skilled in the art, confronted with the problem of balancing minimal heating and high cohesion at high temperature, has proposed a large number of solutions. Thus, it has been proposed to use elastomeric internal filler compositions, i.e., cushion mixes, of relatively low hysteresis, in the form of:
(i) compositions based on natural rubber, pure or in a blend with polybutadiene, the reinforcing filler being a carbon black having a specific surface area preferably less than 110 m2/g and used in an amount of 30 to 35 phr (parts by weight per hundred parts of elastomer);
(ii) compositions based on natural rubber, pure or in a blend with polybutadiene, reinforced with a blend of carbon black and silica, the usual amounts of carbon black being from 30 to 35 phr and those of the silica from 10 to 15 phr;
(iii) compositions based on diene rubber and syndiotactic 1,2-polybutadiene as described in Patent Application JP-A-94/092108;
(iv) compositions based on natural rubber, possibly in a blend with another diene elastomer, comprising carbon black and thermoplastic polymer fibers as described in Patent Application JP-A-95/330960.
The Applicant has discovered that it is possible to obtain a balance between heating and improved cohesion and excellent resistance to the mechanical stresses with high deformation by the use of an elastomeric internal filler composition:
(i) based on natural rubber or synthetic polyisoprene having a majority of cis-1,4 bonds, used pure or in a blend with another diene elastomer,
(ii) reinforced with:
either a carbon black filler used in an amount between 15 phr and 28 phr, and preferably between 20 phr and 25 phr,
or a clear filler selected from among precipitated or pyrogenic silicas comprising SiOH functions at the surface, precipitated aluminas comprising AlOH functions at the surface, a natural or precipitated silicoaluminate comprising at the surface both SiOH and AlOH groups, said clear filler being used in an amount from 15 phr to 40 phr and preferably from 20 phr to 35 phr,
or with a blend of carbon black and clear filler as described above, such that the total amount of filler is between about 15 phr and 50 phr, and that the amount of clear filler in phr is greater than or equal to that of the carbon black in phr minus 5phr.
In the case of using clear filler, it is necessary to use a coupling and/or covering agent selected from among the agents known to the person skilled in the art. Preferred coupling agents include, inter alia, sulphur-containing alkoxysilanes of the bis-(3-trialkoxysilylpropyl) polysulphide type, and among these in particular, bis-(3-triethoxysilylpropyl) tetrasulphide sold by DEGUSSA under the names Si69 for the pure liquid product and X50S for the solid product (blend 50/50 by weight with black N330). Covering agents include a fatty alcohol, an alkylalkoxysilane, such as a hexadecyltrimethoxy- or triethoxysilane sold by DEGUSSA under the names Si116 and Si216 respectively, diphenylguanidine, a polyethylene glycol, or a silicone oil possibly modified by means of OH or alkoxy functions. The covering and/or coupling agent is used in a ratio by weight relative to the filler of between 1/100 and 20/100, and preferably of between 2/100 and 15/100, when the clear filler represents the entire reinforcing filler, and between about 1/100 and 20/100 when the reinforcing filler comprises a blend of carbon black and clear filler.
The elastomeric internal filler compositions or cushion mixes according to the invention are, for example, triangular profiled members separating the carcass reinforcement from the radially inner crown ply, the profiled members located between crown reinforcement plies over their entire width and/or the profiled members separating the ends of the crown plies forming the crown reinforcement.
The diene elastomers which may be used in a blend with natural rubber or a synthetic polyisoprene having a majority of cis-1,4 bonds include a polybutadiene (BR), preferably having a majority of cis-1,4 bonds, a solution or emulsion styrene-butadiene copolymer (SBR), a butadiene-isoprene copolymer (BIR) or, alternatively, a styrene-butadiene-isoprene terpolymer (SBIR). These elastomers may be modified during polymerization or after polymerization by means of branching agents, such as divinylbenzene, or starring agents, such as carbonates, tin halides or silicon halides. Alternatively, the elastromers may be modified by means of functionalizing agents resulting in grafting of oxygenated carbonyl or carboxyl functions or, alternatively, an amine function, such as, for example, by the action of dimethyl- or diethylamino-benzophenone on the chain or at the ends of the chain. In the case of blends of natural rubber or of synthetic polyisoprene having a majority of cis-1,4 bonds with one or more of the diene elastomers referred to above, the natural rubber or the synthetic polyisoprene preferably comprises the majority of the blend, and, more preferably, comprises an amount greater than 70 phr.
When carbon black is used as the sole reinforcing filler, the required properties are obtained using a carbon black, or a blend of carbon blacks, the BET specific surface area of which is between 30 and 160 m2/g, preferably between 90 and 150 m2/g, and the DBP structure of which is between 80 and 160 ml/100 g. Preferably, the amount of black used lies within the range of the values 20 phr and 25 phr. The measurement of BET specific surface area is effected in accordance with the method of BRUNAUER, EMMET and TELLER described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, vol. 60, page 309, February 1938, corresponding to Standard NFT 45007 of November 1987.
When a clear filler is used as the sole reinforcing filler, the hysteresis and cohesion properties are obtained using a precipitated or pyrogenic silica, or a precipitated alumina, or alternatively an alumosilicate of BET specific surface are of between 30 and 260 m2/g. Preferably an amount of filler from 20 to 35 phr is used. Non-limiting examples of this type of filler include the silicas KS404 from Akzo, Ultrasil VN2 or VN3 and BV3370GR from Degussa, Zeopol 8745 from Huber, Zeosil 175MP or Zeosil 1165MP from Rhodia, HL-SIL 2000 from PPG, etc.
In the case of a blend of carbon black with a clear filler, an amount of clear filler from 25 to 40 phr is preferably used.
Other examples of reinforcing fillers having the morphology and the SiOH and/or AlOH surface functions of the silica- and/or alumina-type materials previously described, which can be used according to the invention as partial or total replacement thereof, include carbon blacks modified either during synthesis by adding to the feed oil of the oven a compound of silicon and/or aluminum or after the synthesis by adding an acid to an aqueous suspension of carbon black in a solution of sodium silicate and/or aluminate, so as to cover the surface of the carbon black at least in part with SiOH and/or AlOH functions. As in the case of the above clear fillers, the specific surface area of the filler lies between 30 and 260 m2/g, and the total amount of silica- and/or alumina-type material filler is greater than or equal to 15 phr, preferably greater than 25 phr, and less than or equal to 35 phr. Non-limiting examples of this type of carbon-containing fillers, with SiOH and/or AlOH functions at the surface, include the CSDP-type fillers described in Conference No. 24 of the ACS Meeting, Rubber Division, Anaheim, Calif., May 6th-9th 1997, and of those of Patent Application EPA-0 799 854.
Additional fillers which may also be used to obtain the diene internal filler compositions having the reinforcement and hysteresis properties according to the invention, include blends of one or more carbon blacks with one or more of the other fillers already mentioned having SiOH and/or AlOH functions at the surface, the overall amount of filler being between 15 and 50 phr, preferably between 20 and 45 phr, and the amount of filler with the SiOH and/or AlOH surface functions being greater than or equal to the amount of carbon black minus five.
Finally, with the aim of improving the working and/or the cost of the compositions according to the invention, without the hysteresis and cohesion characteristics being fundamentally changed, the filler or the blends of reinforcing fillers described above may be replaced in part by a less-reinforcing filler, such as a crushed or precipitated calcium carbonate, a kaolin, etc., on the condition that x phr of reinforcing filler is replaced by x+5 parts of less-reinforcing filler, x being less than 15 phr.
The compositions according to the invention may cross-link under the action of sulphur, peroxides or bismaleimides with or without sulphur. They may also contain the other constituents usually used in rubber mixes, such as plasticizers, pigments, antioxidants, and cross-linking accelerators, such as benzothiazole derivatives, diphenylguanidine etc.
The compositions according to the invention may be prepared using known thermomechanical working processes for the constituents in one or more steps. For example, they may be obtained by thermomechanical working in one stage in an internal mixer for 3 to 7 minutes at a speed of rotation of the blades of 50 rpm or in two stages in an internal mixer for 3 to 5 minutes and 2 to 4 minutes respectively, followed by a finishing stage effected at about 80xc2x0 C., during which the sulphur and the accelerator are incorporated, in the case of a sulphur-cross-linked composition.
The invention is illustrated by the following examples, which in no way constitute a limitation to the scope of the invention.
In all the examples, unless indicated otherwise, the compositions are given in parts by weight.
In these examples, which may or may not be in accordance with the invention, the properties of the compositions are evaluated as follows:
Mooney Viscosity
The Mooney viscosity ML (1+4) is measured in accordance with Standard ASTM D1646.
Rheometry
The rheometry measurements are performed by measuring the torque on a Monsanto Model 100S rheometer. They are intended to monitor the vulcanization process by determining the time To in minutes which corresponds to the vulcanization delay and the time T99 in minutes which corresponds to 99% of the maximum torque measured.
Moduli of Elongation
The moduli of elongation are measured at 100% (ME100) and at 300% (ME300) in accordance with Standard ISO 37-1977.
Scott Break Index
These indices are measured at 23xc2x0 C. or 100xc2x0 C. The breaking stress (BS) is determined in MPa and the elongation at break (EB) in %.
Tearability Index
These indices are measured at 100xc2x0 C. The force (TBS) is determined in MPa and the elongation at break (TEB) in % on a test piece of dimensions 10xc3x97105xc3x972.5 mm notched at the center of its length over a depth of 5 mm.
Hysteresis Losses (HL)
The hysteresis losses (HL), or hysteresis, are measured by rebound at 60xc2x0 in accordance with Standard ISO R17667 and are expressed in %.
Tom surface in cm2 After Impact of a Tire Against a Curb
The tire to be tested is first baked at 77xc2x0 C. for 6 weeks in a ventilated oven to simulate aging due to travel.
A heavy vehicle equipped with the tire to be tested hits a curb at very low speed at a fixed angle of less: than 20 degrees. Five passes onto the curb are effected, after which the tire is demounted and then decorticated, and the torn surface measured.
In all the tests, the compositions according to the invention are used in the form of triangular profiled members arranged between the carcass reinforcement and the radially inner crown ply.